Psychological conditions can be very difficult to understand or relate to, both for people who have them and those who do not. Possibly the most central reason for this is because mental illnesses are just that - of the mind, as opposed to something that can be looked at, touched, or analyzed objectively. Furthermore, in terms of having a condition, there is not necessarily an easy way to compare one's psyche to that of others, as opposed to seeing that others have a different height or skin tone.
A brief description of a mental health condition is the presence of thoughts, feelings, or actions (or a combination thereof) in an individual beyond one's control that harmfully affects the individual, others, or their community. A more detailed and specific answer from the American Psychiatric Association (APA) can be found here.
In making a diagnosis, the interpretation and understanding of such involves consideration of numerous aspects regarding an individual, including their life situation and culture. As an example, working to the point where one's life is impacted in certain areas (such as their social life and sleep) can be a symptom of a condition, but factors such as meeting one's basic living needs also need to be taken into account.
The details involved in making a diagnosis are covered in the Diagnostic and Statistical Manual for Mental Disorders (the current edition at the time of this writing is the DSM-5), which is produced by the American Psychiatric Association (APA).
While it may seem straightforward, the aspects involved in diagnosing a psychological condition require an understanding of various subtleties and nuances, both in terms of the individual and the DSM; if one does not have experience related to mental health, one should avoid attempting to make a diagnosis of themself or others. Mental health professionals, such as a psychologist or psychiatrist, spend many years studying and training in relation to this area and, very importantly, personal perspective can influence one's abilities to notice certain aspects, or accept a diagnosis.
To summarize, having known someone a long time, researching conditions in one's spare time, or being a close relative do not qualify one to decide if they or another individual has a psychological condition (or what some may call an "undiagnosed" psychological condition). For reasons including health, reputation, objectivity, and determining relevant (and safe) treatment, an experienced, qualified mental health specialist should be worked with when attempting to determine if a psychological condition (if any) is present.
A key aspect to take into account, however, are the words "work with;" while a provider may have credentials and experiences in the field of mental health, an individual has first-hand experience and understanding of their symptoms and life experiences that no-one else can ever have. In addition, it is healthy, important, and ideal to approach working with one's provider(s) as a team interacting together to address the condition(s) - not simply one individual being told to do something by others.
Should one feel they are not being properly understood, believed, respected, or that the provider is not qualified (in general, or because of their area of experience), one should try to seek a provider they feel more comfortable with (trust being essential to a patient-provider arrangement).